In wireless communications, the reception level (electric field intensity) fluctuates because incoming waves interfere mutually which have passed through a plurality of different paths (multipath) due to reflection, refraction, scattering, and the like caused by structures, natural terrains, and the like (multipath fading). In the line-of-sight communication, since the intensity of incoming waves is relatively low that arrive through paths other than those for direct waves, and that are included in radio waves arriving at the receiving side, there is less fluctuation of transmission characteristics due to a positional relation and the like.
On the other hand, in the non-line-of-sight (NLOS) communication, in which there are obstacles between the transmitting and receiving sides, since no direct waves arrive at the receiving side and there is no difference in the intensities of incoming waves through respective paths, the transmission characteristics fluctuate a lot due to the mutual interference between the incoming waves through the respective paths. For example, the transmission characteristics fluctuate a lot with only a slight variation of the positional relation between the receiving side and the transmitting side which is caused by the movement of the receiving side. Accordingly, in the non-line-of-sight communication, a variety of design systems, considering a fading margin, for example, are adopted as measures against the multipath fading.
In addition, it is known that wireless-transmitting information signals to be transmitted encoded with high error-correcting capability makes it possible to operate even with a small carrier to noise (C/N) ratio. For example, it is known that when an error-correcting code such as the BCH (Bose-Chaudhuri-Hocquenghem) code is applied in the AWGN (Additive White Gaussian Noise) channel, the bit error rate (BER) characteristics concerning the C/N value are improved by an order of magnitude or more compared to a case without the error correction although the information bit rate decreases. That is to say, encoding with the high error-correcting capability decreases the BER, which enables low C/N operations. Low density parity check (LDPC) codes and the like can also be used as the error-correcting code.
In non-line-of-sight communication, down fading is likely to occur at low C/N operation. Down fading is a state in which the receiving electric field intensity keeps falling below a threshold value for a predetermined time or more due to a level fluctuation. In this case, it is likely to get out clock synchronization because a receiving baseband unit cannot regenerate a clock signal. A baseband unit in a wireless receiving device synchronizes a device clock with a received signal (a baseband signal), and performs demodulation processing and decoding processing (error-correcting decoding). Consequently, the baseband unit is prevented from carrying out decoding processing because the occurrence of down fading makes it impossible to perform clock regeneration.
On the other hand, it is possible to avoid the influence of the multipath fading in the non-line-of-sight communication by spreading and transmitting an information signal using the DS-SS (direct sequence spread spectrum) system and performing the spectrum-despreading on the receiving side. For example, the DS-SS system is employed as a secondary modulation in the IEEE (Institute of Electrical and Electronics Engineers) 802.11b, which is the standard for the wireless LAN (local area network) in which the non-line-of-sight communication is performed in the 2.4 GHz band.
In the DS-SS system, the transmitting side performs a secondary modulation with a spreading code on a first-modulated signal (narrow-band modulation wave) for information transmission, and then transmits the signal. For example, in IEEE 802.11b, a first modulation using the phase difference of the carrier wave is performed, that is, the first modulation called DBPSK (Differential Binary Phase Shift Keying) is performed on 1 Mbps signals, DQPSK (Differential Quaternary Phase Shift Keying) on 2 Mbps signals, and CCK (Complementary Code Keying) on 5.5/11 Mbps signals. Then the secondary modulation using the DS-SS system is performed on the signals, which are spread over a 20 MHz bandwidth, for example.
If S(t) represents a narrow-band modulation wave, SpN(t) represents a spreading code composed of 1 and −1, and SSS(t) represents a signal having been spectrum-spread, SSS(t) is given by the following formula.SSS(t)=S(t)×SPN(t)   (Formula 1)
If the bandwidth of S(t) is represented by B and the bandwidth of SPN is represented by BPN, a spreading code is used by which BPN>>B is satisfied. BPN/B is called a spreading ratio. On the receiving side, a received signal SSS(t) spectrum-spread on the transmitting side is multiplied by the same spreading code SPN(t) as that of the transmitting side in the same phase. This enables the original signal S(t) to be restored (spectrum despreading) as shown in formula 2 because SPN(t)×SPN(t)=1.
                                                                                                              S                    SS                                    ⁡                                      (                    t                    )                                                  ×                                                      S                    PN                                    ⁡                                      (                    t                    )                                                              =                            ⁢                                                {                                                            S                      ⁡                                              (                        t                        )                                                              ×                                                                  S                        PN                                            ⁡                                              (                        t                        )                                                                              }                                ×                                                      S                    PN                                    ⁡                                      (                    t                    )                                                                                                                          =                            ⁢                                                S                  ⁡                                      (                    t                    )                                                  ×                                  {                                                                                    S                        PN                                            ⁡                                              (                        t                        )                                                              ×                                                                  S                        PN                                            ⁡                                              (                        t                        )                                                                              }                                                                                                        =                            ⁢                              S                ⁡                                  (                  t                  )                                                                                        (                  Formula          ⁢                                          ⁢          2                )            
It is known that the DS-SS system grows in the interference resistance capability by using a code with strong autocorrelation characteristics as a spreading code. It is necessary, however, to secure a wide occupied bandwidth for data signals if the large-capacity communication is required for the DS-SS wireless communication. In wireless communications in which frequency bands are allocated, it is impractical to achieve a still wider bandwidth corresponding to the large-capacity communication.
Patent Literature 1 discloses a method for improving the detection accuracy of a synchronizing timing by detecting a correlation peak using a correlation threshold value that is set depending on received signals when a S/N (Signal to Noise) ratio of the received signals is low. In the technique disclosed in Patent Literature 1, a default correlation threshold value is generated by multiplying the average value of correlation detection values between spectrum-spread received signals and spreading codes by a parameter varying with a state of synchronization/asynchronization. A final correlation threshold value is generated by comparing the correlation detected value with a default correlation threshold value during a peak detection period, and the generated final correlation threshold value is set in a correlation peak detection unit. The technique disclosed in Patent Literature 1, however, cannot produce a full effect in communications under the multipath fading.
Patent Literature 2 and Patent Literature 3 disclose a technique to shorten a signal acquisition time, and a technique to disperse a harmonic of a clock signal and control a harmonic remaining in a used frequency band, respectively. In addition, Patent Literature 4 discloses a timing reproduction device which extracts a baseband signal from a received signal having been despread and generates a reference clock for a latch timing of the baseband signal based on a power value into which the extracted baseband signal is converted.